Magnetic head employing hard axis thin film inductor

ABSTRACT

A flux-sensitive magnetic head employs, at its back part, a structure having a thin single domain magnetic film coated thereon. A coil wraps around the coated structure, and a direct current is passed through the coil, thereby to apply a hard axis magnetic bias to the film. Signal flux appearing at the head front gap asserts a magnetic force along the hard axis of the film. Contrary to what would be expected, the signal flux causes the inductance of the coil to vary. Such inductance variation may be conveniently detected by (1) connecting the variable inductor into a tank circuit, (2) applying a high frequency ac ripple to the &#34;hard axis&#34; dc bias, and (3) measuring the &#34;hard axis&#34; detuning experienced by the tank circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to magnetic heads and in particular tomagnetic heads of the flux-sensitive type.

2. Description Relative to the Prior Art

The playback of recorded signals from magnetic tape, or the like, usinga conventional magnetic head that is sensitive to flux rate-of-change isdifficult at low signal frequencies, and theoretically impossible for dcsignals, or when there is no relative head-to-tape motion. Varioustechniques have been proposed for sensing tape flux, as opposed to therate-of-change of flux (viz Hall effect devices; flux gate devices;etc.). The invention, as will be described below, employs a "singledomain" thin magnetic film structure, say, one plated with permalloy toa thickness less than about three microns.

Thin film magnetometers have been described in the literature, and in anumber of patents:

"Recent Advances in the Thin Film Inductance Variation Magnetometer", C.J. Bader and C. S. DeRenzi, Intermag, 1947, IEEE;

IEEE Transactions on Magnetics, Vol. Mag-8, #1, March 1972, "MagneticThin-film Magnetometers for Magnetic-Field Measurement", H. Irons and L.Schwee;

U.S. Pat. No. 2,856,581, issued in 1958 to L. Alldredge;

U.S. Pat. No. 3,012,177, issued in 1961 to H. Mortimer;

U.S. Pat. No. 3,239,754, issued in 1966 to W. Odom, Jr.; and

U.S. Pat. No. 3,271,665, issued in 1966 to P. Castro.

Of particular interest is the above-noted article by Bader and DeRenziin which it is stated, categorically, that "variation (of filmcontributed inductance) with hard axis field (H_(y)) . . . with no easyaxis field, is zero for all values of H_(y))" (underlining added).Theoretically this may be the case, but it has been found to beincorrect for real films at various values of hard axis field. This factis used to advantage in apparatus embodying the invention.

SUMMARY OF THE INVENTION

The present invention provides a magnetic head in which the back partthereof is bridged by a thin film structure surrounded by a coil, thehard axis of the film being along the longitudinal axis of the coil. Abias field is applied in the direction of the hard axis (say by passinga direct current through the coil), and an excitation frequency isapplied to the coil (causing field variation at such frequency) also inthe direction of the hard axis of the film. As signal flux is sensed bythe head front gap, it causes the film's hard axis flux to varyaccordingly. A circuit responsive to inductance variations of the coilproduces a signal corresponding to the sensed signal flux. (Note shouldbe taken that in apparatus according to the invention, (1) signal fluxinput, (2) bias flux, (3) excitation flux, and (4) the signal output areall associated with the hard axis of the film.) At the heart of theinvention is "inductance variation as a function of hard axis fieldvariation". According to the published theory (noted above), thefilm-contributed inductance variation is zero for all values of hardaxis field in the absence of an easy axis field. Perhaps due toacceptance of the published theory, the art has avoided the technique ofthe invention. Under actual operating conditions, it has been found thatthe film-contributed inductance of thin magnetic films varies in acontinuous manner from an initial value to a maximum and then toward aminimum value as the hard axis field is increased from an initial valueof zero. Such inductance variation provides a ready means of producingelectrical signals corresponding to the signal flux.

The invention will be further described with reference to the Figures,wherein:

FIGS. 1, 2, 3 and 4 are diagrams useful in describing the invention,

FIG. 5 is a schematic diagram of apparatus embodying the invention,

FIG. 6 is a schematic diagram of apparatus useful in practicing theinvention, and

FIG. 7 is a perspective view of another embodiment of the invention.

Referring now to FIG. 1, which shows the theoretical saturation loopsfor both the easy and hard axes for a perfect single domain thinmagnetic film: The easy axis loop (curve 1) is square and indicates thata field H_(C), applied in the easy axis, the magnetization 4πI switchesfrom one sense to the opposite sense. For example, in the theoreticalmodel, for a thin film coated on a structure of circular cross-section(see FIG. 4), with the film dipole moment oriented in the direction ofthe arrow B, a field of magnitude H_(C) will reverse the direction ofthe film dipole moment. The hard axis loop (FIG. 1, curve 2) is diagonaland indicates that the magnetization 4πI changes linearly with changesin applied field H. In the theoretical model (see FIG. 4), with a field(arrow C) applied axially in relation to the circumferential film, i.e.applied in the direction of the film hard axis, the film dipole momentreorients in the direction of the field C - in proportion to thestrength of field C -- until, at the field strength H_(C), the filmmoment is parallel to field C. It should be emphasized that theforegoing manifestations are theoretical and apply only to the "perfect"single domain model used.

In an actual thin film structure as in FIG. 4, upon which a coil hasbeen wrapped, and in which a hard axis field (arrow C) is applied to thestructure, one would expect an inductance vs hard axis fieldrelationship as indicated by the solid line curve of FIG. 3. (This iswhat was predicted by the model used by Bader and DeRenzi.) That is,with an increasing hard axis field, dB/dH is a constant; thus inductanceshould remain a constant. Note that permeability μ=dB/dH, i.e. μ isproportional to the rate of change of the magnetization with respect tothe magnetizing field. Inductance is directly related to permeability;hence a variation in permeability produces a consequent variation ininductance.

What was found, however, was that the thin film contributed inductancevaried as described previously and as is shown in the dashed-line curveof FIG. 3.

This is explained by the fact that due to dispersion and otherunavoidable defects, a "perfect" single domain thin magnetic film isattainable only in theory, and an actual film re-orients in parts,according to the corresponding curves in FIG. 2, at different values ofapplied field so as to produce a "distribution" type curve instead ofthe theoretical "step" change at the value H_(C).

With the above as background, reference should now be made to FIG. 5which shows a magnetic head 10 in contact with magnetic tape 12. Thehead 10 has magnetic pole pieces 14, 16 which define a transducer gap18, the head being provided with a non-magnetic back bar 20. A thin filmstructure 22 -- say, a wire upon which a thin film was deposited whilethe wire conducted a current -- is connected so as to complete amagnetic circuit from one pole piece to the other. A coil 24 wrapsaround the film structure 22.

Although a hard axis magnetic bias H_(B) may be applied to the filmstructure -- say, by a permanent magnet 26 -- it has been foundconvenient to apply a hard axis bias field to the film by passing adirect current (28) through the coil 24. Such a bias establishes aquiescent inductance for the coil as indicated by the point P of FIG. 3.

A capacitor 30 may be connected across the coil 24 forming a tankcircuit. The capacitor 30 has a capacitance which may be such that theresonant frequency for the tank circuit is below the frequency of, say,an RF excitation (32) applied to the tank circuit. Thus, in the absenceof signal fields (H_(u)) appearing in the head gap 18, the voltagedeveloped across the tank circuit varies at the RF excitation rate . . .this being because the inductance of the coil 24 varies at that rate. Assignal flux appears in the head gap 18, however, the inductance of thecoil varies in accordance with both the RF excitation and the signalflux, and thus the voltage across the tank circuit appears as anamplitude modulated voltage proportional to the signal flux strength,the modulated voltage being detectable by means known to the art.

It is interesting to note that while `bias`, `input`, `excitation`, and`output` are all associated with the hard axis of the film, ambientfields perpendicular to the hard axis have negligible influence on theoperation of apparatus embodying the invention. This will be evidentfrom FIG. 4, which depicts ambient field vectors D₁,2 : any influencethat the field vector D₁ has on the film easy axis is equallycounteracted by the field vector D₂.

Although the apparatus of FIG. 5 depicts an arrangement for AM detectionof signals corresponding to the signal flux (H_(u)) appearing at thehead gap 18, the invention may also be incorporated in other modes ofoperation, including FM. For example, the structure contained within thedashed lines 40 of FIG. 5 may be replaced by the circuit of FIG. 6. Thatis, the coil 24 may comprise the inductor, say, of the tank circuit of aColpitts oscillator, the frequency of which will increase and decreaseas a function of the signal flux H_(u). Such being the case, an FMdemodulator 42, adapted to receive the oscillator output, will providean output voltage proportional to the signal flux H_(u).

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention. For example, although a thin magnetic film coated wireis depicted in FIG. 5, the invention may employ a structure, as depictedin FIG. 7, in which a single domain thin film 44 is deposited on, say, anon-magnetic planar support 46.

What is claimed is:
 1. Magnetic head apparatus comprising:(a) first andsecond magnetic pole pieces disposed to form a transducer gaptherebetween; (b) means supporting an essentially single domainunidirectional magnetic film magnetically coupled to said pole piecesfor completing a magnetic circuit comprising said pole pieces, said gap,and said film, said film being so disposed with respect to said polepieces that flux entering said transducer gap traverses said filmsubstantially in the direction of the hard axis of said film; (c) a coilinductively coupled to the hard axis of said film; (d) means forapplying a magnetic bias along the hard axis of said film in order toeffect a quiescent inductance for said coil, and (e) means coupled tosaid coil for detecting inductance variation thereof as caused by fluxvariations sensed by said gap,whereby flux entering said transducer gapcorrespondingly causes the inductance of said coil to vary detectably.2. The apparatus of claim 1 wherein said means supporting a magneticfilm is an electrically conductive wire having a magnetic film coatedthereon, said film being substantially magnetically coupled to said polepieces.
 3. The apparatus of claim 1 including means for applying anexcitation field, having an alternating component, substantially in thedirection of the coil longitudinal axis.
 4. The apparatus of claim 3including means coupled to said coil for producing a signalcorresponding to inductance variations in said coil.
 5. The apparatus ofclaim 4 wherein said means for producing a signal corresponding toinductance variations comprises capacitance means connected to said coilto form therewith a tank circuitry, and means responsive to voltagevariations associated with said tank circuit.
 6. The apparatus of claim4 wherein said means for producing a signal corresponding to inductancevariations comprises capacitance means connected to said coil to form atank circuit, amplifier means cooperative with said tank circuit to formtherewith an oscillator circuit, and frequency demodulation meanscooperative with the oscillator circuit for producing a signalcorresponding to frequency variations in said oscillator circuit.